Metal and metal ceramic joining are very important processes. These processes are especially important when utilized with shock resistant non-metals such as silicon nitride, molybdenum silicide, silicon carbide and a wide variety of nitrides, carbides, borides, oxynitrides, bor-carbides, carbon nitrides, diamond and other common engineering metallic materials including iron, aluminum, nickel, rare earth and transition metal alloys. The usefulness of many engineering ceramics critically depends upon the ability to successfully join them. The joining of ceramics is considered as a viable alternative technology to the processing and shaping of products with large and/or complex geometric components. Joining is also a viable alternative for the processing of composites consisting of metal and non-metal materials. Similarly, as the functions of tool bits and ceramics become more specific, and their costs remain high, designers are increasingly keen to use ceramics as inserts in otherwise metallic structures. As such, it is absolutely necessary that these ceramic inserts be well bonded to the metal parts. Thus, joining, as part of a manufacturing system, can offer significant advantages for the fabrication of ceramic components, whereas joining is essential for fabricating ceramic-to-metal structures.
The joining of ceramics is a difficult process. Strong, functional and long lasting joints have been realized both in laboratories, and, most importantly, on production floors, with numerous joints presently being in use for applications ranging from engine to biomedical sectors. However, one of the stumbling blocks that still remains is the fabrication of strong refractory joints for, primarily, structural applications. Ceramics such as Si3N4 (Silicon Nitride) are designed for ever higher temperature applications and, as such, SN joints are expected to survive in corrosive environments at high temperatures under stress. In comparison to SN joints, when Ag—Cu—Ti filler metals are employed at brazing temperatures of 800° C., or higher, the joints realized with these brazes can hardly survive beyond 400° C. in oxidizing environments.
The obvious way to increase the refractory capabilities of the joints is by using more refractory filler metals or intelligent ternary and quaternary alloys, thus escalating the manufacturing costs and undermining the materials stabilities. Apparently, what would be ideal is a joining process that allows for joining, at low temperatures and that yields joints that can last at much higher temperatures. Several commercial filler metals, including Au—Pd, Pd—Ni and Ni—Cr based materials can be identified, with solidus temperatures higher than 900° C. However, among these filler metals only the Ni—Cr ones can loosely be classified as active-metal brazes. Many technically important ceramics, including SN, are not wetted by conventional filler metals.
Recent developments, however, have led to a new class of brazes, called active metal brazes. These brazes react chemically with the ceramics to form wettable products on their surfaces and, thus, do not require prior modification of the ceramic surface. However, the service temperatures achievable with the common active brazes that are based on Ag—Cu matrices are low. Nickel brazes with active additives, such as Cr, have been considered as refractory alternatives. Earlier studies have detailed the development of refractory braze alloys. Silicon nitride joints have been made via brazing with an active Au—Ni—V filler metal. In general, brazing with this filler metal is not as easy and straightforward as with the Ag—Cu—Ti active braze alloys. Useful joint strength values (±400 MPa) have been achieved, with slight improvement of the joint strength when bonding in an argon environment. Eutectic, mono-eutectic, liquidus and solidus are characteristics commonly desired in brazes. Of most importance, very promising high temperature properties of the joints were realized; the 900° C. joint strengths were about 100 MPa, while oxidation of the joints at 900° C. for 100 hours did not affect the as bonded strength.